Resistance to the demethylation-inhibiting fungicide propiconazole in Canadian populations of Microdochium nivale

Turfgrass managers have anecdotally reported decreased efficacy of DMI fungicides for the control of Microdochium patch and pink snow mold at golf courses in British Columbia, Canada. Isolates of Microdochium nivale from these locations, along with isolates collected in Ontario, Canada, were tested for their sensitivity to the fungicide propiconazole [1-([2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-yl]methyl)-1,2,4-triazole]. Ontario isolates (47) had values for effective concentration causing 50% growth inhibition (EC₅₀) ranging from <0.001 to 0.89 µg ml^–1. In comparison, British Columbia isolates (50) had an EC₅₀ range of 0.02 to 8.7 µg ml^–1. Sensitivity testing with a discriminatory concentration (0.1 µg ml^–1) of a larger set of isolates revealed that 24% of Ontario isolates (43 of 181) and 77% of British Columbia isolates (55 of 71) exhibited resistance to propiconazole (>50% growth on 0.1 µg ml^–1 compared to non-amended media). Because of the cool, wet climate of coastal British Columbia, turfgrass managers use more applications of fungicides annually, including propiconazole, to control diseases caused by M. nivale, and this has resulted in a greater proportion of isolates being resistant to propiconazole. In contrast, Ontario has a less favorable climate for these diseases, with accordingly fewer fungicide applications directed toward this pathogen and hence less risk of fungicide resistance developing.